The present invention relates to a process for the recovery of organic values from aqueous solutions containing both organic values and scale-forming ions, and more particularly to a process for recovery of polyhydric organic compounds from spent dehydrating agents employed for drying water-containing CO.sub.2. Procedures for drying CO.sub.2, particularly at supercritical or near critical conditions, with electrolytes and/or polyhydric organic compounds such as calcium chloride, polyglycols such as triethlyene glycol, and glycerine are disclosed e.g. in U.S. Pat. Nos. 4,478,612; 4,492,592; 4,492,593; and 4,500,333 (all incorporated by reference).
The recovery of organic values from aqueous solutions containing electrolytes has often been realized commercially by the use of evaporative processing, in which the water is first evaporated, then the desired organic substances are evaporated and recovered. In this procedure the electrolytes concentrate at in the bottom of the evaporation equipment, and when scale-forming electrolytes are present the evaporative process can be hampered by scale formation and resulting loss of heat exchanging efficiency at the surfaces of the associated heat exchanging units.
As the heat transfer becomes ineffective due to fouling, the skin temperatures in the heat exchanger must be increased, which may result in localized "hot spots" and increased propensity of the organic molecules to decompose. The present invention circumvents this problem by first electrodialyzing the feed to remove the electrolytes under conditions Which will not cause precipitation of scale which might otherwise occur in the electrodialysis unit.
The precipitation of scale can be particularly serious at higher pH values during electrodialysis, e.g. in the region above about 9 pH. Thus, the presence of bicarbonate ions can be deleterious, in that at high pH carbonate ions would form which would lead to precipitation of e.g. calcium as the carbonate. Further it has been found that the aqueous solutions resulting from the drying of subterranean CO.sub.2 always contain bicarbonate ion and often contain in addition to calcium, other divalent metal ions such as magnesium and barium, and sulfate ions as well. The formation of e.g. calcium sulfate scale is particularly troublesome as it is difficult to remove from the surface of heat exchange equipment, even after acld washing. In an electrodialysis unit the scale formation can be a serious problem in the area close to the anion exchange membranes. During normal operation, localized conditions in these regions can result in pH values which are two or-three pH units above the pH in the bulk solution. Although localized, these regions can nevertheless cause the chemical equilibrium in the solution undergoing electrodialysis to shift bicarbonate to carbonate ion, thus starting formation of calcium carbonate scale. This in turn facilitates precipitation of other scale forming materials such as e.g. calcium sulfate.
It has now been found that such scale formation in the electodialysis step can be substantially reduced or avoided by introducing into the concentrate side of the electrodialysis unit an aqueous carrier in an amount such that precipitation of the ions present is substantially reduced or avoided.